1. Field of Invention
The present application is related generally to the field of high-fidelity speaker systems where motional feedback signal is used to enhance the performance.
2. Prior Art
(Magnetic) Flux modulation is a modulation of magnetic flux passing through the air gap of the motor structure as a function of applied current. Such modulation is undesirable in that it can cause distortion in the sound reproduced from the speakers. The cause of flux modulation can be explained with reference to FIG. 1, a typical motor structure. In FIG. 1, 1 is the pole piece; 2 is the magnet; 3 is the top plate; 4 is the voice coil; 5 is the back plate; and the space between 1 and 3 is called the air gap, which is also where voice coil 4 resides. The dashed line indicates the magnetic flux direction in the motor structure. The drawn arrow direction is arbitrary as it depends on the polarity of 2. The voice coil also acts as an inductor. That means when a current passes through it, it generates magnetic flux in the pole piece just as an inductor will do. On the other hand, the motor structure is designed to facilitate the passing of magnetic flux through the air gap. Therefore it is designed such that the dashed line loop is the least effort return path for the magnetic flux. Because of this reason, the magnetic flux generated by a current can follow the dashed line loop to complete the least effort return path. And this additional magnetic flux causes the modulation of the magnetic flux.
Flux modulation is getting worse in recent years because high power amplifiers and speakers (>1 kW) are becoming popular. Higher power means higher current and therefore higher flux modulation. And this problem not only affect the drivers used to low frequency (as often referred to as subwoofer), it also affect the drivers used at the higher frequency. In the past, short circuit rings have been placed around the pole piece to reduce the effect of flux modulation, such as one described in paper “Moving-coil Loudspeaker Topology as an Indicator of Linear Excursion Capability” by Mark R. Gander in Journal of AES Vol 29 No 1/2 1981 January/February, pp 10-26. The issue with short circuit rings is the physical size of the ring may have to be huge in order to be effective, and that increases the cost. And smaller rings are not effective at reducing flux modulation at low frequency. Other similar techniques include U.S. Pat. Nos. 5,815,587 and 5,151,943.
All of the above-mentioned techniques address the flux modulation from a magnetic circuit's operating point of view. The present invention addresses this issue using a nonlinear feedback system with the feedback signal as a nonlinear function of two components: 1) current through the driver voice coil (the main voice coil that converts the electrical energy to mechanical energy), and 2) the motional feedback signal which is derived from a sensing coil wound on the same former as the main voice coil, which is also referred to as the driver coil. In other words, the present invention is completely orthogonal with the usage of short-circuit rings and can be applied at the same time.
Other patents of possible interest include U.S. Pat. No. 5,764,781 issued Jun. 9, 1998, U.S. Pat. No. 6,104,817 issued Aug. 15, 2000, and U.S. Pat. No. 7,215,590 issued May 8, 2007.
Nonlinear feedback systems have been proposed in the past to linearize the BL profile over a large travel distance of the voice coil. One technique is U.S. Pat. No. 5,542,001, which taught a method of linearizing the back-emf motional signal by introducing a multiplier, integrator, and correction generator in an inner feedback loop within the motional feedback path. That is, there is a local feedback loop inside the global feedback loop.
Yet, another technique was proposed when A. J. M. Kaizer examined the issue of correcting the nonlinearity of electrodynamic loudspeakers; and the result was published in the paper “Modeling of the Nonlinear Response of an Electrodynamic Loudspeaker by a Volterra Series Expansion”, appeared in Journal of Audio Engineer Society, vol. 35, no 6, 1987 June, page 421-433. The paper proposed a voltage drive and a current drive 2nd order distortion reduction circuits. However, both circuits operate on a feedforward principle instead of a feedback principle and A. J. M. Kaizer subsequently obtained U.S. Pat. No. 4,709,391. The main difference between a feedforward system and a feedback system is that the former needs a forward prediction of how the system will perform and generates a forward correction term based on the prediction to achieve a lower distortion. As a result, the unit-to-unit variation and change of operation condition can render the techniques less effective.
The objectives of the above-mentioned non-linear feedback techniques were to linearize the BL profile and therefore they are insufficient in addressing the issue of flux modulation.